I. Field of the Invention
The present invention relates to digital communications. More particularly, the present invention relates to a novel and improved method and system for interfacing a digital telephone system with a standard public switched telephone network to avoid double vocoding.
II. Description of the Related Art
Digital wireless telephone systems provide telephone service via the use radio of frequency (RF) signals and digital signal processing techniques. The use of RF signals provides the advantage of mobility over traditional wire based telephone systems, and reduces the amount of infrastructure necessary to implement the telephone system. The use of digital signal processing techniques allows telephone calls to be transmitted more efficiently and thereby allows digital wireless telephone systems to carry greater numbers of telephone calls over a given amount of RF bandwidth when compared to a non-digital wireless telephone systems. Maximizing the efficiency with which a wireless telephone system utilizes RF bandwidth is desirable because the amount of RF bandwidth available is a limiting factor as to the number of telephone calls the particular wireless telephone system can carry.
One of the most common types of digital signal processing techniques utilized within a digital wireless telephone systems is vocoding. Vocoding incorporates the use of selective data elimination and data compression to convert a first digital representation of audio information, usually generated by sampling, into a second digital representation that requires less data. Selective data elimination is the act of eliminating some of the information encoded by the data in a way that still allows a comprehensible version of the original audio information to be generated. By eliminating data, and compressing the data which remains, vocoding substantially reduces the amount of digital data that must be transmitted across the RF interface of the wireless telephone system for a telephone call and thereby increases the total number of calls that the wireless telephone systems can carry.
In general, the data associated with a telephone call involving a digital wireless telephone system is introduced into a wire based telephone system for routing to the receiving subscriber unit which is usually part of a wire based telephone system. Wire based telephone systems have traditionally performed the routing function and generally store the information necessary to complete each telephone call. Before being introduced into the wire based telephone system, however, the vocoded data from the digital wireless telephone system must be devocoded using signal processing resources within the digital wireless telephone system. This is because wire based telephone systems generally lack the resources necessary for devocoding the data so that it can be understood by the end user. Once introduced into the wire based telephone system the devocoded data is routed to the receiving subscriber unit.
If the receiving subscriber unit is also part of a digital wireless telephone system, referred to herein as a "wireless subscriber unit," the devocoded data is routed into the associated digital wireless telephone system. Upon being introduced into the digital wireless telephone system the devocoded data is revocoded so that it may be processed further by the digital wireless telephone system. Because selective data elimination causes some audio information to be lost, however, vocoding audio information that has previously been vocoded and devocoded substantially degrades the quality of the audio information that is ultimately produced. Therefore, this present system of routing telephone calls between two wireless subscriber units causes undesirable degradation of audio information. While this "double vocoding" is presently a limited problem because the majority of telephone calls involve at least one wire based subscriber unit, the number of digital wireless subscribers to digital wireless subscriber telephone calls is steadily increasing as the availability of wireless telephone service also increases. Therefore, there is a need for an improved method and system for processing telephone calls where both the initiating and receiving subscribers are part of a wireless telephone system.
Various other changes in telecommunications technology, in addition to the increasing availability of wireless telephone service, have also altered the way people use their telephones. Two of the most prevalent examples of such changes are conference calling and call waiting. Conference calling allows multiple subscribers to communicate with each other simultaneously, and requires that outgoing audio information from two or more subscriber units be combined before being supplied to a receiving subscriber unit. This combining adds additional complexity to the processing of calls involving two or more wireless subscriber units because vocoded data cannot readily be combined in this manner. The call waiting feature allows a single subscriber to alternately communicate with two other subscribers. This can give rise to the situation where a wireless subscriber must interface with both another wireless subscriber and a wire based subscriber further complicating the processing of a wireless subscriber to a wireless subscriber call. These features are very popular services with telephone service subscribers, however, and are a substantial source of revenue for telephone service providers. Therefore, it is desirable to have any improved system and method for processing telephone calls involving two or more wireless subscriber units that also accommodate the use of conference calling and call waiting.